The present invention relates in general to control systems and in particular to a new and useful cascaded control system for a universal pressure boiler.
A known method of steam temperature control for universal pressure boilers, as illustrated in FIG. 1, utilizes a furnace gas temperature (FGT) 2, a primary superheater outlet temperature (PSHOT) 3, and a secondary superheater outlet temperature (SSHOT) 4 as primary controlled variables. This known control scheme utilizes a feedforward program based on a feedwater temperature error (FWTE) 22, a secondary superheater inlet temperature (SSHIT) 23, a secondary superheater outlet temperature error (SSHOTE) (not shown) and a primary super heater outlet temperature error (PSHOTE) 21.
The known steam temperature control system illustrated in FIG. 1 is devised to maintain a secondary superheater outlet temperature at a set point in order to account for transient disturbances within the universal pressure boiler. Currently, the conventional control system for the universal pressure boiler utilizes the furnace gas temperature transmitter 2, the secondary superheater outlet temperature transmitter 3 and a primary superheater outlet temperature transmitter 4 as the primary controlled variables. Unit load demand 20 is input into a function generator 10 having a maximum gas temperature. A difference unit 12 performs a subtracting action of the furnace gas temperature 2 and the set program of furnace which is in function generator 10 gas temperature and provides this difference to a low select auctioneer 13 for taking the lower value of the two.
Unit load demand 20 is input into a second function generator 110 for controlling the set program of secondary superheater outlet temperature. A second difference unit 112 performs a subtraction action on the secondary superheater outlet temperature 3 and the unit load demand 20 from the set program of the secondary superheater outlet temperature function generator 110. This difference is provided to a high select auctioneer 14 for taking the higher value of the two. Unit load demand 20 is input to a third function generator 210 for providing the desired primary superheater outlet temperature. This difference or primary superheater outlet temperature error 21 is provided to the high select auctioneer 14. The primary superheater outlet temperature error 21 is also provided to the low select auctioneer 13 in conjunction with the difference from the first difference unit 12.
The low select auctioneer 13 provides the lower value i.e. the difference from the difference unit 12 or the primary superheater outlet temperature error 21, to a derivative action unit 15. The lower value from the low select auctioneer 13 is also provided to a summer 17 and a transfer action unit 16. Derivative action unit 15 performs a rate function upon the value from the low select auctioneer 13. This rate provided by the derivative action unit 15 is summed along with the value from the low select auctioneer 13 by the summer 17. After a summing action is performed by the summer 17, this value is provided to a second summer 117.
The high select auctioneer 14 takes the greater value from either the primary superheater outlet temperature error 21 or the value provided by the second difference unit 112. The greater value is provided by the high select auctioneer 14 to the transfer action unit 16. The value from the high select auctioneer 14 is also provided to a second derivative action unit 115 for performing a rate function which is in turn provided to the second summer 117.
The transfer action unit 16 performs a transfer action of the value from the low select auctioneer 13 and the high select auctioneer 14 which constitutes a low load or bypass selection and once-through operation. Transfer action unit 16 transfers these values to an integral action control unit 18 which performs an integral function on these values and provides the result to the second summer 117.
Second summer 117 performs a summing action on values from the second derivative action unit 115, the integral action control unit 18, the first summer 17, a feedwater temperature error 22 and a secondary superheater inlet temperature 23 supplied through a third derivative action unit to 215.
After performing the summing action at the second summer 117, this value is provided to a multiplier 19 for performing a multiplying action. The unit load demand 20 is provided through a fourth function generator 310 to the multiplier 19 for multiplication with the value from the second summer 117.
The value from the multiplier 19 is provided to an inverse proportional action unit 11 and a fourth summer 317. Fuel flow demand 24 is also provided to the third summer 217 which in turn performs a summing action on the loss from the proportional action unit 11 and the fuel flow demand 24 for determining a firing rate 30 for the universal pressure boiler.
The fourth summer 317 takes the value from the multiplier 19 along with a feed water flow demand 25 for performing a summing action. Due to the summing action performed on these Values, a feedwater flow rate 31 for the universal pressure boiler is determined.
The conventional steam temperature control strategy, as described above, is divided into main regions: a low load or bypass region and an operating or high load region. In the low load region, which is generally the low range before a transfer is conducted from a flash tank operation or drum boiler type to a once-through operation, the furnace gas temperature 2 is used as the controlled variable. The set point is the load base 20 which is characterized for temperature. In the low load region, the primary superheater outlet temperature error 21 is used as an overriding signal for the furnace gas temperature error when the maximum temperature operating limit is reached near the minimum feedwater flow load. The overriding circuit of the conventional control system also provides protection for the superheater tubes.
The operating or high load region of the universal pressure boiler covers the load point range from a once-through operation to a full load operation. In the high-load region, it is necessary to use the primary superheater outlet temperature 4 and/or the secondary superheater outlet temperature 3 as the controlled variables. At this stage, temperature probes used in conjunction with the universal pressure boiler are retracted in order to render the furnace gas temperature control 3 as a non-factor.
The set point for the high load region is generated as a function of load characterized as steam temperature. A secondary superheater outlet temperature error is created by comparing the secondary superheater outlet temperature 3 to the programmed set point. This error is limited by the primary superheater outlet temperature error 21 which acts as an override circuit and a protector for the superheaters in case of extreme temperature excursions.
During a high load range, this scheme provides for transient conditions by using transient factors. One such transient factor is the feedwater temperature error 22 wherein an increase of this error implies a reduction in the superheater temperature because for any given steam flow, less fuel is fired into the boiler and less gas passes over the superheaters.
Another transient factor is a secondary super-heater inlet temperature kicker which is a signal used for adjusting the feedwater to fuel ratio which is caused by a change in the operating condition reflected at the secondary superheater inlet temperature 23 before the secondary superheater outlet temperature 3. Another transient factor is the secondary superheater outlet temperature error wherein its rate of change is indexed as a function of load and is used to correct the feedwater rate and firing rate.
There are several problems involved with employing the conventional steam temperature control as described above. First there is instability within the boiler at low load operations when controlled by the furnace gas temperature variable 2. Second, there is a lack of control and unaccountability for the tremendous dead time that exists at low loads due to the high heat absorption. Third, there is limited adaptive control capability for the system since the system controls only the final element i.e. the secondary superheater outlet temperature 3. Fourth, there is a slow or sluggish response in the operating range and overcompensation or undercompensation can occur due to the numerous kickers that are used as anticipatory signals. Fifth, the secondary superheater outlet temperature 3 is used as a kicker but in reality this signal is a feed-back and if not used with moderation, can create a continuous cycling problem for the system.